On Jan 08 2019, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Mon, Jan 7, 2019 at 10:05 PM Nikolaus Rath <Nikolaus@xxxxxxxx> wrote:
>>
>> On Jan 07 2019, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> > On Wed, Dec 26, 2018 at 10:44 PM Nikolaus Rath <Nikolaus@xxxxxxxx> wrote:
>> >>
>> >> Hi,
>> >>
>> >> I am seeing relatively regular occurences of
>> >>
>> >> $ sudo dmesg | tail
>> >> [21929.138815] fuse: trying to steal weird page
>> >> [21929.138821] page=00000000a7dd2617 index=64 flags=17fffc0000000ad,
>> >> count=1, mapcount=0, mapping= (null)
>> >> [21930.647338] fuse: trying to steal weird page
>> >> [21930.647345] page=00000000a07f32af index=2848
>> >> flags=17fffc0000000ad, count=1, mapcount=0, mapping= (null)
>> >> [21932.338873] fuse: trying to steal weird page
>> >> [21932.338879] page=0000000067e3a012 index=64 flags=17fffc0000000ad,
>> >> count=1, mapcount=0, mapping= (null)
>> >> [21933.930703] fuse: trying to steal weird page
>> >> [21933.930710] page=00000000046feb25 index=845
>> >> flags=17fffc0000000ad, count=1, mapcount=0, mapping= (null)
>> >> [21936.163174] fuse: trying to steal weird page
>> >> [21936.163180] page=00000000fb80fe27 index=0 flags=17fffc0000000ad,
>> >> count=1, mapcount=0, mapping= (null)
>> >
>> > The page has the PG_dity and PG_waiters flags set which are
>> > incompatible with stealing. page_cache_pipe_buf_steal() does
>> > apparently filter out dirty ones, so it's not a regular file that we
>> > are trying to streal the page from. So the question is: what is the
>> > source of the splice()?
>>
>> Hmm. I think it has to be a regular file. But as I mentioned in my other
>> email, I did have a race condition where fd's were closed
>> incorrectly. Is it possible that this also triggered the above,
>> i.e. that the fd was closed sometime during splice?
>
> Close during a syscall that uses the fd is not an issue, because a ref
> to the file is acquired. So the race is between the close() and the
> internal fget(); if the close() wins then fget() will fail and the
> syscall will return EBADF. If the fget() wins, then the syscall can
> run normally despite the fact that the fd was closed.
>
> Can you tell me what filesystem is the regular file (the one being
> spliced into fuse) is on?
It's ext4.
> It actually has to be a regular file, since AFAIK nothing else has
> dirty pages. It could be using something other than
> page_cache_pipe_buf_steal(), or there's some other mechanism that lets
> the page be dirtied after being unmapped, though that looks
> impossible...
I can't meaningfully comment on that, sorry...
Best,
-Nikolaus
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